FIG. 5 is a sectional view showing an example of a conventional image forming apparatus. In FIG. 5, the electrophotographic image forming apparatus has a freely rotatable photosensitive drum 1 serving as an image carrier. The photosensitive drum 1 is uniformly charged by a primary charger 2, and exposed to an information signal by a light-emitting element 3 such as a laser to form an electrostatic latent image. The electrostatic latent image is visualized into a toner image by a developing device 4. The toner image is transferred by a transfer charger 9 onto a transfer paper sheet 14 conveyed by a transfer sheet convey sheet. The toner image is then fixed by a fixing device 11 to obtain a permanent image. Toner left on the photosensitive drum 1 after transfer is removed by a cleaning device 8.
In the image forming apparatus, the developing device 4 contains toner and carrier serving as a developer. Screws 43 and 44 stir the developer to charge the toner. A development method using such two-component developer causes the following phenomenon. The toner in the developer is consumed by development, replenished from a toner cartridge 5, and replaced by outputting an image. When images (low-coverage-rate images) which hardly consume toner are successively output, toner is replaced only little by little. Toner stays in the developing device for a long time, is rubbed between a blade 42 and a developing sleeve 41 for a long time, and stirred in a developing chamber 47 and stirring chamber 48.
The toner in the developer which is repetitively rubbed and stirred for a long time changes to an irregular shape or suffers offset of the particle diameter distribution. Further, an external additive such as a titanium oxide particle which is added to the developer in order to improve the flowability becomes buried in the toner surface. This results in degradation such as poor flowability of the developer, failing to obtain an image which keeps a desired image quality.
Charges of toner stabilize upon rubbing several times. However, repetitive rubbing gradually charges toner to exceed a predetermined charge amount. As charges of toner increase, the amount of toner attached to a latent image which is formed on the photosensitive drum 1 and is equal in potential difference to the developing sleeve 41 decreases from an initial state. This appears as degradation of the image quality such as low density upon outputting an image or conspicuous graininess at a low-density portion.
As another development method, various dry monocomponent development methods have been proposed and commercially available. One of these methods is impression (contact) development. Impression development has many advantages such as easy simplification and downsizing of the apparatus because this development does not require any magnetic material (carrier). In addition, impression development can form a color image by using nonmagnetic toner.
FIG. 6 shows a developing unit using impression (contact) development. In impression (contact) development, an electrostatic latent image is developed by pressing the surface of a toner carrier against the electrostatic latent image or bringing the surface into contact with the image. As the toner carrier, an elastic, conductive developing roller must be adopted. In order to obtain a known development electrode effect or bias effect, a conductive layer is formed on or near the surface of the developing roller, and a bias voltage can be applied as needed. Toner is charged by triboelectrification between the toner carrier and a developing blade for forming a toner layer.
In the above-described impression (contact) development, the image quality degrades as the number of formed images increases. Degradation of the image quality is mainly caused by toner deterioration, and main causes are rubbing between the developing roller and the surface of the image carrier and rubbing between the developing roller and the developing blade. This phenomenon occurs when toner near the developing roller is hardly consumed and rubbed for a long time. In particular, the phenomenon becomes prominent when many low-coverage-rate images are printed.
To solve these problems, there is proposed a method of forcedly consuming a predetermined amount of toner by development considering the relationship between the number of turns of the developer carrier or the print count and the coverage rate.
As a simple method, a predetermined amount of toner is consumed when the number of turns of the developer carrier reaches a predetermined value. According to this method, however, a predetermined amount of toner is consumed even when a large amount of toner is consumed. Forced toner consumption operation is executed even under conditions in which no problem occurs, wastefully consuming toner. To prevent this, a predetermined amount of toner is consumed only when the toner amount is equal to or smaller than a given threshold (coverage rate) which is calculated from the number of turns of the developer carrier or the print count and the sum of laser exposure amounts for one image of video count data.
The flow of a detailed process is shown in FIG. 3. The number of output images is set (step S301), and when image formation starts (step S302), a CPU 30 loads input image data (step S303), and calculates an image ratio (step S304). After one image is output (steps S305 and S306), whether the image ratio is equal to or smaller than a predetermined value (e.g., 5%) is determined (step S307). If YES in step S307, a predetermined amount of toner is consumed (step S308). A latent image is formed in the entire region along the axial direction of a photosensitive drum at a laser irradiation amount of FFH so as to consume an amount of toner corresponding to a predetermined value (corresponding to an image ratio of 5%). The predetermined amount may be constant or changed depending on the image ratio of an output image. When the predetermined amount is changed, it is preferable to consume an amount of toner corresponding to an image ratio of 5% or more including the image ratio of the output image. After the end of consumption and replenishment of toner, whether the number of output images reaches a designated count is checked (S309). If NO in step S309, the process is repeated from loading of image data (from step S303); if YES, image formation ends (step S310).
This sequence need not always be executed. The sequence is executed only when the print count or the number of turns of the developer carrier reaches a predetermined value. This can reduce the number of forced toner consumption operations.
By the above-described sequence, a predetermined amount of toner is always replaced, and excessive rubbing of the same toner can be prevented. As a result, excessive charging and deterioration of toner are suppressed.
In the conventional scheme described above, the following phenomena occur. The deterioration degree or charged state of toner sometimes greatly changes owing to preparation variations in the developer of toner and carrier or the difference in the characteristic of the regulation blade or developer carrier. When the developing unit is designed as a unit detachable from the image forming apparatus main body, the characteristic of the developer or each part may vary between units. If forced toner consumption operation is executed on the basis of a predetermined threshold or consumption amount, toner is excessively consumed, or an image deteriorates due to insufficient consumption.